Reverse osmosis composite membrane and method for manufacturing reverse osmosis composite membrane

ABSTRACT

A reverse osmosis composite membrane includes: a porous support; and a reverse osmosis membrane arranged on the porous support and containing a crosslinked polyamide and carbon nanotubes. The reverse osmosis membrane contains the carbon nanotubes that are disentangled in the crosslinked polyamide. A distribution of closest distances between the carbon nanotubes in the reverse osmosis membrane has a peak that is within a range of a thickness of the reverse osmosis membrane, and a half width of the peak is equal to or less than the thickness of the reverse osmosis membrane.

TECHNICAL FIELD

The present invention relates to a reverse osmosis composite membraneusing carbon nanotubes and a method of manufacturing the membrane.

BACKGROUND ART

In order to cope with global water shortage and water contamination, awater treatment technology involving using a reverse osmosis membrane(RO membrane) has been attracting attention. Of such membranes, areverse osmosis membrane using an aromatic polyamide that can removeeven an ion component, such as salt, with a pore diameter of 1 nm orless has been most widely spread in seawater desalination plants.

In a water treatment involving utilizing a reverse osmosis membrane, apermeate flux and desalination performance (NaCl rejection rate) reducewith time owing to the adhesion of dirt to the reverse osmosis membrane.Accordingly, the permeate flux and the desalination performance needs tobe restored by periodically washing the reverse osmosis membrane. Awashing method desired for the washing of the reverse osmosis membraneis washing with an aqueous solution containing a hypochlorite or activechlorine having an oxidizing power because the washing is excellent indecomposing and removing properties for a protein component and thelike, microbicidal property, and sterilizing property.

However, the reverse osmosis membrane using the aromatic polyamide haslow resistance to chlorine having an oxidizing power, and hence aportion in contact with chlorine deteriorates. Accordingly, the membraneinvolves a problem in that its desalination performance extremelyreduces after washing with oxidizing chlorine relative to that beforethe washing.

Reverse osmosis membranes using carbon nanotubes have also been proposed(Non Patent Literatures 1 to 3). However, each of the proposals isinferior in performance to a reverse osmosis membrane currentlyavailable on the market, and shows no dramatic improvement in chlorineresistance.

CITATION LIST Non Patent Literature

-   NPL 1: Hee Joong Kim and seven others, “High-Performance Reverse    Osmosis CNT/Polyamide Nanocomposite Membrane by Controlled    Interfacial Interactions,” ACS Appl. Mater Interfaces 2014, 6,    2819-2829-   NPL 2: Hee Dae Lee and three others, “Experimental Evidence of Rapid    Water Transport through Carbon Nanotubes Embedded in Polymeric    Desalination Membranes,” Small, Volume 10, Issue 13, pages    2653-2660, Jul. 9, 2014-   NPL 3: Haiyang Zhao and five others, “Improving the performance of    polyamide reverse osmosis membrane by incorporation of modified    multi-walled carbon nanotubes” Journal of Membrane Science, 450,    2014, 249-256

SUMMARY OF INVENTION Technical Problem

In view of the foregoing, an object of the present invention is toprovide a reverse osmosis composite membrane excellent in chlorineresistance and a method of manufacturing the reverse osmosis compositemembrane.

Solution to Problem Application Example 1

A reverse osmosis composite membrane according to this ApplicationExample includes:

a porous support; and

a reverse osmosis membrane arranged on the porous support and containinga crosslinked polyamide and carbon nanotubes,

the reverse osmosis membrane containing disentangled carbon nanotubes inthe crosslinked polyamide;

a distribution of closest distances between the carbon nanotubes in thereverse osmosis membrane having a peak that is within a range of athickness of the reverse osmosis membrane; and

a half-width of the peak being equal to or less than the thickness ofthe reverse osmosis membrane.

Application Example 2

In the above reverse osmosis composite membrane, the disentangled carbonnanotubes may form a continuous three-dimensional structure through thecrosslinked polyamide.

Application Example 3

In the above reverse osmosis composite membrane,

the crosslinked polyamide may be a crosslinked aromatic polyamide; and

the reverse osmosis membrane may contain a molecularly-orientedcrosslinked aromatic polyamide configured to cover the carbon nanotubes.

Application Example 4

In the above reverse osmosis composite membrane, a content of the carbonnanotubes in the reverse osmosis membrane may be 5 mass % or more and 30mass % or less.

Application Example 5

In the above reverse osmosis composite membrane, the distribution may bea normal distribution.

Application Example 6

In the above reverse osmosis composite membrane, the carbon nanotubesmay have an average diameter of 5 nm or more and 30 nm or less.

Application Example 7

In the above reverse osmosis composite membrane, themolecularly-oriented crosslinked aromatic polyamide may form a layer ona surface of each of the carbon nanotubes, the layer having a thicknessof 1 nm or more and 50 nm or less.

Application Example 8

In the above reverse osmosis composite membrane,

the reverse osmosis composite membrane may have a permeate flux of 0.8m³/(m²·day·MPa) or more and a NaCl rejection rate of 95% or more when anaqueous solution of NaCl having a pH of 7, a temperature of 23° C., anda concentration of 2,000 ppm is supplied at an operating pressure of1.55 MPa; and

the reverse osmosis composite membrane may have a reduction rate of theNaCl rejection rate of less than 10% after having been immersed in anaqueous solution of sodium hypochlorite having a pH of 9±0.5, atemperature of 23° C., and a concentration of 200 ppm for 24 hours.

Application Example 9

In the above reverse osmosis composite membrane, the reverse osmosiscomposite membrane may have a reduction rate of a permeate flux of lessthan 35% after having been brought into contact with an aqueous solutioncontaining bovine serum albumin at a concentration of 200 ppm for 72hours.

Application Example 10

A method of manufacturing a reverse osmosis composite membrane accordingto this Application Example includes:

bringing a mixed liquid containing carbon nanotubes, water, and an aminecomponent into contact with a porous support; and

then subjecting the amine component in the mixed liquid adhering to theporous support to a crosslinking reaction,

the mixed liquid being produced through a step of pressurizing andcompressing an aqueous solution containing the carbon nanotubes whileflowing the aqueous solution, followed by release or reduction of apressure to return a volume of the aqueous solution to an originalvolume to mix the carbon nanotubes.

Application Example 11

In the above method of manufacturing a reverse osmosis compositemembrane,

a content of the amine component in the mixed liquid may be 1.0 mass %or more and 3.0 mass % or less; and

a content of the carbon nanotubes in the mixed liquid may be 0.11 mass %or more and 1.3 mass % or less.

Application Example 12

In the above method of manufacturing a reverse osmosis compositemembrane, the amine component may be an aromatic amine.

Application Example 13

In the above method of manufacturing a reverse osmosis compositemembrane, the aqueous solution containing the carbon nanotubes mayfurther contain a surfactant.

Advantageous Effects of Invention

The reverse osmosis composite membrane according to the presentinvention enables washing with an aqueous solution containing oxidizingchlorine. In addition, the method of manufacturing a reverse osmosiscomposite membrane according to the present invention enables themanufacture of a reverse osmosis composite membrane that can be washedwith an aqueous solution containing oxidizing chlorine.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a longitudinal sectional view for schematically illustrating areverse osmosis composite membrane 100.

FIG. 2 is a plan view for schematically illustrating a smooth surface ofa reverse osmosis membrane 104 observed with a scanning electronmicroscope.

FIG. 3 is a graph for illustrating the distribution of closest distancesbetween carbon nanotubes.

FIG. 4 is a schematic diagram of a carbon nanotube 110 in the reverseosmosis membrane 104.

FIG. 5 is a permeate flux-NaCl rejection rate graph.

FIG. 6 is a graph of a NaCl rejection rate and a permeate flux againstthe product of a chlorine concentration and an immersion time.

FIG. 7 is a photograph of a tom surface of the reverse osmosis compositemembrane 100 of Example 1 obtained with a scanning electron microscope.

FIG. 8 is a photograph of the carbon nanotube 110 of the reverse osmosiscomposite membrane 100 of Example 1 obtained with a transmissionelectron microscope.

FIG. 9 is a transmission electron microscope photograph of a crosslinkedaromatic polyamide in a reverse osmosis membrane free of any carbonnanotube.

FIG. 10 is a photograph of a reverse osmosis composite membrane ofComparative Example 2.

FIG. 11 is a photograph obtained by enlarging part of FIG. 10.

FIG. 12 is a graph for illustrating a change in permeate flux when areverse osmosis composite membrane is brought into contact with anaqueous solution containing bovine serum albumin at a concentration of200 ppm for 96 hours.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention are described in detail below withreference to the drawings.

A. Reverse Osmosis Composite Membrane

A reverse osmosis composite membrane according to one embodiment of thepresent invention includes: a porous support; and a reverse osmosismembrane arranged on the porous support, the reverse osmosis membranecontaining a crosslinked polyamide and carbon nanotubes. The reverseosmosis membrane contains disentangled carbon nanotubes in thecrosslinked polyamide. A distribution of closest distances between thecarbon nanotubes in the reverse osmosis membrane has a peak within arange of a thickness of the reverse osmosis membrane, and a half-widthof the peak is equal to or less than the thickness of the reverseosmosis membrane.

FIG. 1 is a longitudinal sectional view for schematically illustrating areverse osmosis composite membrane 100.

In the reverse osmosis composite membrane 100, a reverse osmosismembrane 104 is arranged on a porous support 102. At least one surfaceof the porous support 102 is covered with the reverse osmosis membrane104. The reverse osmosis membrane 104 contains a crosslinked polyamide120 (description is given below by taking a crosslinked aromaticpolyamide as an example, but the polyamide is not limited thereto) andcarbon nanotubes 110. The entirety of the surface (observed with amicroscope) of the reverse osmosis membrane 104 is covered with thecrosslinked aromatic polyamide 120.

The reverse osmosis membrane 104 contains the disentangled carbonnanotubes 110 in the crosslinked aromatic polyamide 120. The crosslinkedaromatic polyamide 120 serves as a matrix, and a gap between thedisentangled carbon nanotubes 110 adjacent to each other is filled withthe crosslinked aromatic polyamide 120. Raw materials for the carbonnanotubes are typically in such an aggregate state as to be brought intocontact with each other by an intermolecular force, thereby forming anagglomerate. However, the carbon nanotubes are disentangled from theagglomerate by a step to be described later, and hence the carbonnanotubes 110 are brought into such a disentangled state as to bedispersed in the crosslinked aromatic polyamide 120. The fact that thecarbon nanotubes 110 are disentangled in the crosslinked aromaticpolyamide 120 may be confirmed by the distribution of closest distancesbetween the carbon nanotubes 110 in the reverse osmosis membrane 104.

The closest distances between the carbon nanotubes may be measured byobservation with a scanning electron microscope. Specifically, a thinfilm-like test piece in which the surface of the reverse osmosismembrane 104 is turned into a smooth surface is cut out through cuttingalong the surface of the reverse osmosis composite membrane 100 by acryo-microtome method (e.g., cutting at a position indicated by thearrow illustrated in the left side of FIG. 1), and the smooth surface(reverse osmosis membrane 104) of the test piece is observed with thescanning electron microscope.

FIG. 2 is a plan view for schematically illustrating the smooth surfaceof the reverse osmosis membrane 104 observed with the scanning electronmicroscope. When the smooth surface of the reverse osmosis membrane 104is observed with the scanning electron microscope, the cut portions ofthe carbon nanotubes 110 scattered in the crosslinked aromatic polyamide120 are found as illustrated in FIG. 2. In FIG. 2, the cut portions ofthe carbon nanotubes 110 are illustrated by black points. In the reverseosmosis membrane 104, the closest distances between the carbon nanotubesare each measured not as an interval between the surfaces of the carbonnanotubes but as a distance between the centers of the cut surfaces ofthe carbon nanotubes.

A method of measuring the closest distances is specifically describedwith reference to FIG. 2.

First, such an image of the smooth surface of the reverse osmosismembrane 104 photographed with the scanning electron microscope asillustrated in FIG. 2 is captured in a computer.

Next, a measurer displays the image on the screen of the computer, andacquires coordinates on the image for each of the cut portions of apredetermined number (20,000) of the carbon nanotubes 110 illustrated byblack points in FIG. 2 present in a predetermined area (a measurementarea of 441 μm²).

Next, when the coordinates of a predetermined number of black pointsclose to each other are acquired, any other black point at the closestdistance from each black point is found, and a distance between the twopoints is determined for each black point. For example, the coordinatesof a black point at a position closest to the coordinates of a carbonnanotube 110 a in FIG. 2 out of a plurality of black points presentaround the carbon nanotube 110 a are the coordinates of a carbonnanotube 110 b, and a distance between the two points is a closestdistance L in the carbon nanotube 110 a. The operation of determiningthe closest distance L from the distance between the two points isperformed for each black point. The operation of finding the coordinatesof any other black point closest from the coordinates of a black pointin the image, the operation of measuring a distance between the twopoints, and the operation of determining the closest distance L may beautomatically analyzed and processed with the computer.

The distribution of the closest distances between the carbon nanotubes110 is created from the measurement results as a graph in which theresults are plotted against an axis of abscissa indicating a closestdistance (nm) and an axis of ordinate indicating the number ofmeasurement points (frequency). A measurement area and the number ofmeasurement points in the test piece are 441 μm² and 20,000,respectively. When the measurement area and the number of measurementpoints in the test piece are 200 μm² or more and 10,000 or more,respectively, and the carbon nanotubes 110 close to each other aresubjected to the measurement without omission, a distribution with whichwhether or not the carbon nanotubes 110 are disentangled can be judgedcan be obtained. However, it is suitable that the measurement area be400 μm² or more, and the number of measurement points be 20,000 or more.

In the present invention, a state in which the carbon nanotubes 110 inthe reverse osmosis membrane 104 are disentangled is a state in whichthe distribution of the closest distances between the carbon nanotubes110 in the reverse osmosis membrane 104 shows a peak within the range ofthe thickness of the reverse osmosis membrane 104, and the half-width ofthe peak is equal to or less than the thickness of the reverse osmosismembrane 104.

The distribution of the closest distances between the carbon nanotubesis described with reference to FIG. 3. FIG. 3 is a graph forillustrating an example of the distribution of the closest distancesbetween the carbon nanotubes, and the measurement of the distributionwas made on the disentangled carbon nanotubes. A sample (disentangledsample) in which the carbon nanotubes are disentangled used in themeasurement is a test piece cut out of a reverse osmosis membraneproduced under the conditions of Example 1 to be described later. Asillustrated in FIG. 3, the distribution of the closest distances betweenthe disentangled carbon nanotubes has a peak within the range of thethickness (e.g., 100 nm) of the reverse osmosis membrane as representedby triangle marks. In addition, the half-width of the peak is equal toor less than the thickness of the reverse osmosis membrane. In addition,the distribution of the closest distances between the disentangledcarbon nanotubes is a normal distribution. When the carbon nanotubes arenot sufficiently disentangled and hence an agglomerate of the carbonnanotubes is present, as represented by circle marks in FIG. 3, thedistribution of closest distances therebetween does not have any clearpeak within the range of the thickness of the reverse osmosis membrane,and does not show any normal distribution. In the example illustrated inFIG. 3, a sample (entangled sample) in which an agglomerate is presentand carbon nanotubes are not disentangled is a test piece cut out of areverse osmosis membrane produced under the conditions of ComparativeExample 2 to be described later. An interval between carbon nanotubes inan agglomerate (in the description of the present application, the term“agglomerate” refers to an agglomerate having a maximum diameter of 50nm or more) cannot be measured. This is because when the reverse osmosismembrane 104 is cut by a cryo-microtome method, the membrane is cutwhile an agglomerate is avoided, and hence no agglomerate can beobserved on a smooth surface of the membrane.

The measurement results of FIG. 3 are obtained by performing themeasurement after cutting out a thin film-like test piece in which thesurface of the reverse osmosis membrane 104 is turned into a smoothsurface as illustrated in FIG. 1. However, even when the reverse osmosismembrane 104 is cut in its thickness direction and the resultant sectionis subjected to the measurement, a similar distribution is basicallyobtained. This is because the carbon nanotubes 110 are distributed in asubstantially three-dimensionally isotropic manner.

In the reverse osmosis membrane containing the disentangled carbonnanotubes, the carbon nanotubes are dispersed at a relatively highconcentration (high blending ratio), and hence the closest distancesbetween the carbon nanotubes seldom become larger than the thickness ofthe reverse osmosis membrane. Most of the closest distances between thecarbon nanotubes are equal to or less than the thickness of the reverseosmosis membrane, and hence the half-width of the peak in thedistribution of the closest distances is equal to or less than thethickness of the reverse osmosis membrane, and the position of the peakfalls within the range of the thickness of the reverse osmosis membrane.

In addition, when the carbon nanotubes are not disentangled, anagglomerate occurs, and hence in a site where no agglomerate is present,the concentration of the carbon nanotubes is low and the carbonnanotubes are widely dispersed. Accordingly, many closest distancesequal to or more than the thickness of the reverse osmosis membrane arepresent, and hence a wide distribution like that of the entangled sampleof FIG. 3 is obtained and the number of such measurement points thatclosest distances therebetween are equal to or less than the thicknessof the reverse osmosis membrane is small.

The distribution of the closest distances between the carbon nanotubesin the reverse osmosis membrane may be such a normal distribution asillustrated in FIG. 3. This is because when the carbon nanotubes in thereverse osmosis membrane are disentangled, the variability of thedistribution of the closest distances becomes smaller and hence thedistribution of the closest distances between the carbon nanotubes showsa normal distribution. The term “normal distribution” as used hereinalso includes a distribution approximate to the normal distribution. Inaddition, the distribution of the closest distances between the carbonnanotubes in the reverse osmosis membrane may be a Poisson distributionor a Lorentz distribution.

The experience of the measurement of the samples of Examples has shownthat the closest distances of a sample in which carbon nanotubes aredisentangled show a normal distribution having a mean of 20 nm or moreand 80 nm or less, and a standard deviation a of 20 nm or more and 75 nmor less.

In such a graph showing the distribution of the closest distancesbetween the carbon nanotubes as illustrated in FIG. 3, as theconcentration of the carbon nanotubes in the reverse osmosis membranebecomes higher, the peak to appear shifts to a left side; in contrast,as the concentration of the carbon nanotubes becomes lower, the peak toappear shifts to a right side.

Next, a three-dimensional structure is described. As illustrated in FIG.1, in the reverse osmosis membrane 104, the disentangled carbonnanotubes 110 can form a three-dimensional structure through thecrosslinked aromatic polyamide. The term “three-dimensional structure”refers to a structure in which the carbon nanotubes 110 dispersed in thecrosslinked aromatic polyamide of the reverse osmosis membrane 104 areconnected to each other at a portion where the nanotubes intersect eachother through the crosslinked aromatic polyamide to bethree-dimensionally continuous. In other words, the three-dimensionalstructure may be paraphrased as a structure in which the carbonnanotubes 110 are formed into a three-dimensionally spreading network.The crosslinked aromatic polyamide adheres to the surfaces of the carbonnanotubes 110 in the three-dimensional structure, and in a portion wherethe carbon nanotubes 110 are connected to each other, the carbonnanotubes 110 are close to each other while being distant from eachother by the thickness of the crosslinked aromatic polyamide adhering tothe nanotubes. When the many disentangled carbon nanotubes 110 aredispersed in the entirety of the reverse osmosis membrane 104, thethree-dimensional structure forms a steric continuous structure in theentirety of the membrane.

Next, molecular orientation is described. FIG. 4 is a view forschematically illustrating the carbon nanotube 110 in the reverseosmosis membrane 104. The carbon nanotube 110 in the reverse osmosismembrane 104 may include a molecularly-oriented crosslinked aromaticpolyamide layer 112 configured to cover its surface. The crosslinkedaromatic polyamide in contact with, or close to, the disentangled carbonnanotubes 110 may be molecularly oriented. When the molecularly-orientedcrosslinked aromatic polyamide layer 112 is formed so as to cover thecarbon nanotube 110, the crosslinked aromatic polyamide layer 112 ismechanically reinforced by the carbon nanotube 110, and hence the peelstrength of the crosslinked aromatic polyamide layer 112 to the carbonnanotube 110 is improved. Further, when the content of the carbonnanotubes 110 increases, the crosslinked aromatic polyamide layers 112that are in contact with, or close to, the carbon nanotubes 110 to bemolecularly oriented approach or overlap each other. Then, the volume ofthe molecularly-oriented crosslinked aromatic polyamide layers 112 inthe entirety of the reverse osmosis membrane 104 increases, and as aresult, mechanical strength and chemical resistance in the reverseosmosis membrane 104 are improved. As a result, peeling resistance andoxidation resistance can be enhanced while the reverse osmosis compositemembrane 100 (FIG. 1) has high desalination performance. Here, the term“peeling resistance” means that the crosslinked aromatic polyamide layer112 hardly peels from the carbon nanotube 110, and the term “oxidationresistance” means difficulty in deterioration by oxidizing chlorine,i.e., chlorine resistance. In order that the crosslinked aromaticpolyamide layer 112 may be necessarily in a state of being adjacent to acarbon nanotube in the reverse osmosis membrane 104, in the case of, forexample, the carbon nanotubes 110 having an average diameter of 5 nm ormore and 30 nm or less, their content is desirably 10 mass % or more.

Under such content of the carbon nanotubes, even when the crosslinkedaromatic polyamide that is not molecularly oriented, which is presentbetween the crosslinked aromatic polyamide layers 112 adjacent to eachother, slightly remains in the reverse osmosis membrane 104, the portionthat is not molecularly oriented and the molecularly-orientedcrosslinked aromatic polyamide layers 112 are integrated and are hencemechanically reinforced. Accordingly, the entirety of the membrane maybe able to have relatively high chlorine resistance.

The molecular orientation of the crosslinked aromatic polyamide layer112 may be confirmed by subjecting the reverse osmosis membrane 104 toelectron diffraction analysis with a transmission electron microscope.

As is apparent from the foregoing description, a state in which thedistribution of the closest distances L of the carbon nanotubes 110 hasa peak within the range of the thickness of the reverse osmosis membrane104 (FIG. 3) is a state in which the carbon nanotubes 110 are arrangedso as to be close to each other in any place of the reverse osmosismembrane 104. The carbon nanotubes 110 arranged so as to be close toeach other are linked to each other by the molecularly-orientedcrosslinked aromatic polyamide layers 112 (FIG. 4) to form athree-dimensional structure having high mechanical strength in thereverse osmosis membrane 104. The crosslinked aromatic polyamide layer112 configured to cover the periphery of the carbon nanotube 110 mayhave high peeling resistance to the carbon nanotube 110. In addition,the molecularly-oriented crosslinked aromatic polyamide layers 112 arepresent in substantially the entirety of the reverse osmosis membrane104 together with the carbon nanotubes 110, and hence the oxidationresistance (chlorine resistance) is enhanced while desalinationperformance exhibited by the crosslinked aromatic polyamide is secured.

It is desired that the reverse osmosis membrane 104 be substantiallyfree of any agglomerate of the carbon nanotubes 110. When an agglomerateis present in the reverse osmosis membrane 104, the agglomerate portionserves as a structural defect to impair the strength of the membrane. Inaddition, the reverse osmosis membrane 104 having many agglomerates isliable to be deteriorated by washing with oxidizing chlorine because aregion formed only of the crosslinked aromatic polyamide in which thecarbon nanotubes 110 are not present, in particular a region formed ofthe crosslinked aromatic polyamide that is not molecularly oriented iswidely present between an agglomerate and an adjacent agglomerate.Further, in the reverse osmosis membrane 104 having many agglomerates,the crosslinked aromatic polyamide does not enter any agglomerate, andhence the desalination performance is impaired.

The content of the carbon nanotubes 110 in the reverse osmosis membrane104 illustrated in FIG. 1 may be 5 mass % or more and 30 mass % or less.Further, the content of the carbon nanotubes 110 in the reverse osmosismembrane 104 may be 10 mass % or more and 30 mass % or less, and inparticular, may be 12.5 mass % or more and 30 mass % or less. When thecontent of the carbon nanotubes 110 in the reverse osmosis membrane 104is 5 mass % or more, a three-dimensional structure can be formed in theentirety of the reverse osmosis membrane 104. In particular, when thecontent of the carbon nanotubes 110 in the reverse osmosis membrane 104is 12.5 mass % or more, most of the closest distances L are equal to orless than the thickness of the reverse osmosis membrane 104. Inaddition, when the content of the carbon nanotubes 110 in the reverseosmosis membrane 104 is 30 mass % or less, the carbon nanotubes 110 canbe covered with the aromatic polyamide.

The thickness of the reverse osmosis membrane 104 may be 50 nm or moreand 1,000 nm or less, and may be 100 nm or more and 500 nm or less. Whenthe thickness of the reverse osmosis membrane 104 is 50 nm or more, thethree-dimensional structure of the carbon nanotubes 110 can be formed,and when the thickness is 1,000 nm or less, a practical permeate fluxmay be obtained.

The reverse osmosis composite membrane 100 can be used even at arelatively high operating pressure because the membrane is excellent inpressure resistance by virtue of a reinforcing effect exhibited by thethree-dimensional structure of the carbon nanotubes 110. The fact thatthe operating pressure can be increased contributes to an increase inpermeate flux.

Examples of the kinds of solutions to be separated with the reverseosmosis composite membrane 100 include high-concentration brine,seawater, and concentrated seawater (desalination).

The permeate flux, NaCl rejection rate, increase rate of the permeateflux, and reduction rate of the NaCl rejection rate of the reverseosmosis composite membrane 100 are described later.

A-1. Carbon Nanotubes

The average diameter (fiber diameter) of the carbon nanotubes may be 5nm or more and 30 nm or less. The thickness of a commercial reverseosmosis composite membrane is 100 nm or more and 500 nm or less, andhence thin carbon nanotubes having an average diameter of 30 nm or lessare preferred, and carbon nanotubes having an average diameter of 5 nmor more are preferred in terms of the ease of handling in adisentangling step to be described later.

In the detailed description of the present invention, the averagediameter and average length of the carbon nanotubes may be obtained by:measuring diameters and lengths at 200 or more sites selected from animage photographed with an electron microscope at a magnification of,for example, 5,000 times (the magnification may be appropriately changedin accordance with the sizes of the carbon nanotubes); and calculatingtheir arithmetic average values.

The carbon nanotubes may be subjected to, for example, an oxidationtreatment in order that reactivity with a liquid at each of theirsurfaces may be improved.

The carbon nanotubes may be so-called carbon nanotubes each having atubular shape obtained by winding one sheet surface (graphene sheet) ofgraphite having a carbon hexagonal net surface, and may be multi-walledcarbon nanotubes (MWCNT).

The carbon nanotubes having an average diameter of 5 nm or more and 30nm or less may be, for example, NC-7000 manufactured by Nanocyl.

A carbon material partially having a carbon nanotube structure may alsobe used. The carbon nanotubes may be called by the name of “graphitefibril nanotube” or “vapor-grown carbon fiber” in addition to the nameof“carbon nanotube.”

The carbon nanotubes may be obtained by chemical vapor deposition. Thechemical vapor deposition is also referred to as “catalytic chemicalvapor deposition (CCVD),” and is a method involving subjecting a gas,such as a hydrocarbon, to vapor-phase thermal cracking in the presenceof a metal-based catalyst to manufacture the carbon nanotubes. Thechemical vapor deposition is described in more detail. For example,there may be used a floating reaction method in which an organiccompound, such as benzene or toluene, is used as a raw material, anorganic transition metal compound, such as ferrocene or nickelocene, isused as a metal-based catalyst, and the compounds are introduced into areaction furnace set at a reaction temperature as high as, for example,400° C. or more and 1,000° C. or less together with a carrier gas toproduce the carbon nanotubes in a floating state or on the wall of thereaction furnace, or a substrate reaction method in whichmetal-containing particles carried in advance on a ceramics, such asalumina or magnesium oxide, are brought into contact with acarbon-containing compound at high temperature to produce the carbonnanotubes on a substrate.

The carbon nanotubes having an average diameter of 5 nm or more and 30nm or less may be obtained by the substrate reaction method, and carbonnanotubes having an average diameter of more than 30 nm and 110 nm orless may be obtained by the floating reaction method.

The diameters of the carbon nanotubes may be regulated by, for example,the sizes of the metal-containing particles and a reaction time. Thenitrogen adsorption specific surface area of the carbon nanotubes havingan average diameter of 5 nm or more and 30 nm or less may be 10 m²/g ormore and 500 m²/g or less. Further, the nitrogen adsorption specificsurface area may be 100 m²/g or more and 350 m²/g or less, and inparticular, may be 150 m²/g or more and 300 m²/g or less.

A-2. Polyamide

The polyamide may be an aromatic polyamide. The polyamide in the reverseosmosis membrane is a crosslinked body.

The aromatic polyamide contains an aromatic amine component. Thearomatic polyamide may be a wholly aromatic polyamide. The aromaticamine is preferably at least one aromatic polyfunctional amine selectedfrom the group consisting of m-phenylenediamine, p-phenylenediamine,1,3,5-triaminobenzene, 1,2,4-triaminobenzene, 3,5-diaminobenzoic acid,2,4-diaminotoluene, 2,4-diaminoanisole, amidole, xylylenediamine,N-methyl-m-phenylenediamine, and N-methyl-p-phenylenediamine. Thearomatic polyfunctional amines may be used alone or in combinationthereof.

The crosslinked aromatic polyamide may have a functional group selectedfrom the group consisting of COO⁻, NH₄ ⁺, and COOH.

The reverse osmosis membrane may contain a molecularly-orientedcrosslinked aromatic polyamide configured to cover the carbon nanotubes.The crosslinked aromatic polyamide forms the crosslinked aromaticpolyamide layer 112 configured to cover the surface of the carbonnanotube 110 as illustrated in FIG. 4. At least the crosslinked aromaticpolyamide layer 112 adjacent to the surface of the carbon nanotube 110is molecularly oriented in the layer. The molecular orientation refersto the alignment of the fine crystal or polymer chain of the crosslinkedaromatic polyamide in a certain direction, and in this description, alsoincludes a polyamide showing a tendency of molecular orientation. Themolecular orientation may be confirmed by, for example, an electrondiffraction method in a transmission electron microscope. When a halopattern appears as a result of the electron diffraction method, nomolecular orientation occurs. When the molecular orientation occurs, thehalo pattern is separated to provide a non-annular pattern, and when themolecular orientation is significant, a spot appears. In thisdescription, such a polyamide that its halo pattern obtained by theelectron diffraction method is separated to show a non-annular patternis also regarded as being molecularly oriented.

In FIG. 4, the diameter of the carbon nanotube 110 is represented by D1and the thickness of the crosslinked aromatic polyamide layer 112 isrepresented by D2. The molecularly-oriented crosslinked aromaticpolyamide layer 112 may have a thickness D2 of 1 nm or more and 50 nm orless on the surface of the carbon nanotube 110. The crosslinked aromaticpolyamide layer 112 may be molecularly oriented as a result of a π-πinteraction between a π-electron of the carbon nanotube 110 and aπ-electron of the crosslinked aromatic polyamide. The crosslinkedaromatic polyamide layer 112 adhering to the periphery of the carbonnanotube 110 has high resistance to washing with an aqueous solutioncontaining oxidizing chlorine.

A-3. Porous Support

The porous support 102 illustrated in FIG. 1 is arranged for impartingmechanical strength to the reverse osmosis membrane 104. The poroussupport 102 may be substantially free of any separation performance.

The porous support 102 has fine pores over a range from its frontsurface to its rear surface. Polysulfone, cellulose acetate, polyvinylchloride, polyacrylonitrile, polyphenylene sulfide, or polyphenylenesulfide sulfone may be used as the porous support 102. The polysulfoneis suitable for the porous support 102 because of its high chemical,mechanical, and thermal stabilities.

A-4. Permeate Flux and NaCl Rejection Rate

The reverse osmosis composite membrane can maintain a high permeate fluxand a high NaCl rejection rate even after washing with an aqueoussolution containing oxidizing chlorine. This is because the crosslinkedaromatic polyamide of the reverse osmosis membrane is hardly damaged bythe washing with the aqueous solution containing oxidizing chlorine, andhence a crosslinked aromatic polyamide layer present around a carbonnanotube hardly falls from the surface of the carbon nanotube.

It is probably because the reverse osmosis membrane includes thethree-dimensional structure that the carbon nanotubes form in adisentangled state and through the crosslinked aromatic polyamide thatthe reverse osmosis membrane maintains a high NaCl rejection rate evenafter the washing with the aqueous solution containing oxidizingchlorine. In addition, it is also probably because the reverse osmosismembrane contains the molecularly-oriented crosslinked aromaticpolyamide configured to cover the carbon nanotubes.

The reverse osmosis composite membrane may have a permeate flux of 0.8m³/(m²·day·MPa) or more and a NaCl rejection rate of 95% or more when anaqueous solution of NaCl having a pH of 7, a temperature of 23° C., anda concentration of 2,000 ppm is supplied at an operating pressure of1.55 MPa, and may have a NaCl rejection rate of 95% or more and 99.9% orless. The permeate flux and NaCl rejection rate of the reverse osmosiscomposite membrane may be measured with, for example, a cross-flow testcell apparatus having an effective area of 140 cm² (TOSC Co., Ltd., SEPACFII).

The reverse osmosis composite membrane may have a reduction rate of theNaCl rejection rate of less than 10% after having been immersed in anaqueous solution of sodium hypochlorite having a pH of 9±0.5, atemperature of 23° C., and a concentration of 200 ppm for 24 hours, andmay have a reduction rate of the NaCl rejection rate of 0% or more andless than 10%.

In addition, a reduction rate R_(r) (%) of the NaCl rejection rate isthe rate at which a NaCl rejection rate R₂ after chlorine washingreduces relative to a NaCl rejection rate R₁ before the chlorinewashing, and may be obtained by the equation “R_(r)=100·(R₁−R₂)/R₁.”When the reduction rate of the NaCl rejection rate R₂ is less than 10%,the deterioration of the reverse osmosis membrane may be suppressed.

A-5. Antifouling Property

The crosslinked aromatic polyamide is molecularly oriented, and thecarbon nanotubes are in a disentangled state and form thethree-dimensional structure, and hence the reverse osmosis compositemembrane may have a reduction rate of its permeate flux of 10% or moreand less than 40% after having been brought into contact with an aqueoussolution containing bovine serum albumin at a concentration of 200 ppmfor 72 hours, and may have a reduction rate of 10% or more and less than35%.

The reverse osmosis composite membrane may be used by being incorporatedinto, for example, a spiral, tubular, or plate-and-frame module, orbeing incorporated into such module after hollow yarns have beenbundled.

B. Method of Manufacturing Reverse Osmosis Composite Membrane

A method of manufacturing a reverse osmosis composite membrane accordingto one embodiment of the present invention is a method of manufacturinga reverse osmosis composite membrane including: bringing a mixed liquidcontaining carbon nanotubes, water, and an amine component into contactwith a porous support; and then subjecting the amine component in themixed liquid adhering to the porous support to a crosslinking reaction,in which the mixed liquid containing the carbon nanotubes, the water,and the amine component is produced through a step of pressurizing anaqueous solution containing the carbon nanotubes while flowing theaqueous solution, followed by the reduction of the pressure to mix thecarbon nanotubes.

The step of obtaining the mixed liquid may include, for example, a stepof mixing a first aqueous solution containing the amine component and asecond aqueous solution containing disentangled carbon nanotubes toprovide a third aqueous solution containing the amine component and thecarbon nanotubes.

B-1. Step of Obtaining Third Aqueous Solution

The first aqueous solution contains water and the amine component. Atleast one kind may be selected from the aromatic amines described in theA-2 as the amine component.

The second aqueous solution contains water and the carbon nanotubes. Inthe second aqueous solution, the carbon nanotubes can be present whilebeing uniformly dispersed in the entirety of the aqueous solution in adisentangled state. The second aqueous solution is obtained from a firstmixing step and a second mixing step.

In the first mixing step, a predetermined amount of the water and thecarbon nanotubes loaded into a container may be stirred manually or maybe stirred with a known stirrer. An aqueous solution obtained in thefirst mixing step is in a state in which the carbon nanotubes aredistributed alone in a particulate manner in the water. Carbon nanotubesused in a related-art reverse osmosis membrane are stirred with anultrasonic stirrer or the like. Accordingly, their agglomerate ispresent as subdivided agglomerates in an aqueous solution, and is notdisentangled. After the first mixing step, the subsequent second mixingstep is performed on the aqueous solution.

The second mixing step includes a step of pressurizing the aqueoussolution containing the carbon nanotubes obtained in the first mixingstep, while flowing the aqueous solution, to compress the aqueoussolution, followed by the release or reduction of the pressure of theaqueous solution to return the volume of the aqueous solution to itsoriginal volume. The second mixing step is repeatedly performed aplurality of times. For example, a triple roll may be used in the secondmixing step. A roll nip between the respective rolls may be set to 0.001mm or more and 0.01 mm or less. Although the triple roll is used here,the number of rolls is not particularly limited, and a plurality ofrolls, such as a twin roll, may be used, and in the case, the aqueoussolution may be kneaded at the same roll nip.

In the second mixing step, a speed ratio between the rolls may be 1.2 ormore and 9.0 or less, and may be 3.0 or more and less than 9.0. This isbecause as the speed ratio between the rolls increases, a shear force onthe aqueous solution enlarges to act as a force for separating thecarbon nanotubes. The phrase “speed ratio between the rolls” as usedherein refers to a speed ratio between adjacent rolls.

In the second mixing step, the peripheral speed of each roll may be 0.1m/s or more and 2.0 m/s or less, and may be 0.1 m/s or more and 1.5 m/sor less. This is because as the peripheral speed of each roll increases,even the aqueous solution can be kneaded by utilizing elasticity. Thephrase “peripheral speed of each roll” as used herein refers to thespeed of the surface of the roll.

The aqueous solution supplied to the rolls enters an extremely narrownip between the rolls. The aqueous solution is pressurized while beingflowed by the speed ratio between the rolls, and a predetermined volumethereof is sequentially supplied to the nip and compressed at the nip sothat the volume may reduce. After that, once the aqueous solution passesthe nip, its pressure is released or reduced, and hence its volume isreturned to the original volume. Then, in association with the return ofthe volume, the carbon nanotubes largely flow and agglomerated carbonnanotubes disentangle. When the series of steps is repeatedly performeda plurality of times, the disentanglement of the carbon nanotubes in theaqueous solution advances and hence the second aqueous solution can beobtained. The second mixing step may be performed for, for example, 3minutes or more and 10 minutes or less. For example, when the series ofsteps is counted as one time, the second mixing step may be performed 10times or more and 30 times or less.

In addition, the second mixing step may be performed while thetemperature of the aqueous solution obtained in the first mixing step isset within the range of from 0° C. or more to 60° C. or less. Further,the second mixing step may be performed while the temperature of theaqueous solution obtained in the first mixing step is set within therange of from 15° C. or more to 50° C. or less. The second mixing stepis preferably performed at as low temperature as possible because thestep is performed by utilizing the bulk modulus of water. The bulkmodulus is in a proportional relationship with a Young's modulus, and isthe reciprocal of a compressibility. This is because the Young's modulusreduces with increasing temperature and the compressibility increaseswith increasing temperature, and hence the bulk modulus also reduceswith increasing temperature. Therefore, the temperature of the aqueoussolution may be set to 60° C. or less, and may be set to 50° C. or less.From the viewpoint of productivity, the temperature of the aqueoussolution may be set to 0° C. or more, and may be 15° C. or more. This isbecause when the temperature of a roll is low, a problem with, forexample, dew condensation in the roll occurs.

The second mixing step is not limited to the kneading with rolls, suchas the triple roll, and any other method may be adopted as long as themethod is a kneading method by which the volume of the aqueous solutioncan be returned after having been compressed. For example, there may beused a dispersion apparatus configured to: pressurize the aqueoussolution, while flowing the aqueous solution, to compress the aqueoussolution to cause cavitation or turbulence; and then abruptly reduce thepressure.

By virtue of a shear force obtained in the second mixing step, a highshear force acts on the water, and the agglomerated carbon nanotubes arerepeatedly passed through the rolls to be separated from each othergradually, disentangled, and dispersed in the aqueous solution, andhence the dispersibility and dispersion stability (difficulty with whichthe carbon nanotubes agglomerate again) of the carbon nanotubes areexcellent.

In addition, the second aqueous solution may further contain asurfactant for maintaining a state in which the carbon nanotubes aredisentangled. Examples of the surfactant include ionic surfactants andnonionic surfactants. Examples of the ionic surfactants include: anionicsurfactants, such as sulfuric acid ester-type, phosphoric acidester-type, and sulfone acid-type surfactants; cationic surfactants,such as a quaternary ammonium salt-type surfactant; and amphotericsurfactants, such as alkyl betaine-type, amide betaine-type, and amineoxide-type surfactants. Further, examples of the nonionic surfactantsinclude fatty acid esters and sorbitan fatty acid esters.

The third aqueous solution may be obtained by mixing the first aqueoussolution and the second aqueous solution containing the disentangledcarbon nanotubes. The content of the aromatic amine in the third aqueoussolution is adjusted to 1.0 mass % or more and 3.0 mass % or less, andthe content of the carbon nanotubes therein is adjusted to 0.11 mass %or more and 1.3 mass % or less. The content of the aromatic amine in thethird aqueous solution is preferably set within the range because of thefollowing reasons: when the content is less than 1.0 mass %, a crosslinkdensity is not sufficient and hence a salt rejection rate is hardlyobtained; and when the content is more than 3.0 mass %, the amount of anunreacted residual amine increases and hence concern about its elutionfrom the membrane grows. In addition, the content of the carbonnanotubes in the third aqueous solution is preferably set within therange because of the following reasons: when the content is less than0.11 mass %, a three-dimensional structure is not formed in the entiretyof the polyamide and hence chlorine resistance is hardly obtained; andwhen the content is more than 1.3 mass %, the peeling of a crosslinkedaromatic polyamide membrane from the porous support is liable to occur.

B-2. Step of Obtaining Reverse Osmosis Composite Membrane

A step of obtaining the reverse osmosis composite membrane involves:bringing the third aqueous solution obtained as described above intocontact with the porous support; and then subjecting the aromatic aminein the third aqueous solution adhering to the porous support to thecrosslinking reaction.

The third aqueous solution is brought into contact with the poroussupport by applying the solution to the support to impregnate thesupport with the solution. After that, a solution containing acrosslinking agent is further applied onto the third aqueous solution,and a heat treatment is performed to cause a polycondensation reactionat an interface between both the solutions so that the aromatic aminemay be crosslinked to form a reverse osmosis membrane. Thus, the reverseosmosis composite membrane described in the “A. Reverse OsmosisComposite Membrane” can be produced.

For example, an organic solvent solution containing an acid chloridecomponent, such as trimesoyl chloride, terephthaloyl chloride,isophthaloyl chloride, or biphenyldicarboxylic acid chloride, may beused as the crosslinking agent.

Examples of the applications of the reverse osmosis composite membraneinclude a treatment prior to the desalination of seawater or brine, asterilization treatment process for food-washing water, and apretreatment sterilization process for industrial water or domesticwater because its reverse osmosis membrane has excellent chlorineresistance. Further examples of the applications of the reverse osmosiscomposite membrane include a food industry wastewater treatment, anindustrial process wastewater treatment, and a RO pretreatment foractivated sludge-treated water because the reverse osmosis membrane isexcellent in antifouling property.

In the present invention, part of its construction may be omitted, orthe respective embodiments or modification examples may be combined tothe extent that features and effects described in the presentapplication are maintained.

The present invention includes substantially the same construction asthe construction described in the embodiments (a construction having thesame functions, methods, and results, or a construction having the sameobject and effects). In addition, the present invention includes aconstruction obtained by replacing an unessential portion of theconstruction described in the embodiments. In addition, the presentinvention includes a construction exhibiting the same action and effectas those of the construction described in the embodiments, or aconstruction that can achieve the same object as that of the foregoingconstruction. In addition, the present invention includes a constructionobtained by adding a known technology to the construction described inthe embodiments.

EXAMPLES

Examples of the present invention are described below, but the presentinvention is not limited thereto.

(1) Production of Sample of Example 1 (1-1) Production of Porous Support

An N,N-dimethylformamide solution containing 13 mass % of polysulfone(hereinafter referred to as “liquid A”) and an N,N-dimethylformamidesolution containing 20 mass % of polysulfone (hereinafter referred to as“liquid B”) were each prepared by holding the mixture of the solvent andthe solute under heating at 90° C. for 2 hours while stirring themixture.

The respective prepared liquids were each cooled to room temperature,and were supplied to separate extruders to be subjected tohigh-precision filtration. After that, the respective filtered liquidswere simultaneously cast onto a long-fiber nonwoven fabric formed ofpolyethylene terephthalate fibers (yarn diameter: 1 decitex, thickness:about 90 μm, air permeability: 1.3 cc/cm²/sec) through a double slit dieso that the liquid A had a thickness of 110 μm and the liquid B had athickness of 90 μm. 2.8 Seconds after the casting, the resultant wasimmersed in pure water and washed for 5 minutes. Thus, a porous supportwas obtained.

(1-2) Production of Third Aqueous Solution

488 g of a first aqueous solution obtained by adding 478 g of distilledwater to 10 g of m-phenylenediamine, and stirring and mixing thecontents with a magnetic stirrer, and 12 g of a second aqueous solutioncontaining disentangled carbon nanotubes were stirred and mixed with amagnetic stirrer to provide 500 g of a third aqueous solution containing2.0 mass % of m-phenylenediamine and 0.4 mass % of the carbon nanotubes.

Here, the second aqueous solution is produced through a step ofpressurizing an aqueous solution containing the carbon nanotubes whileflowing the aqueous solution, followed by the reduction of the pressureto uniformly mix the carbon nanotubes. Specifically, the second aqueoussolution was obtained by: stirring 10 g of distilled water and 2 g ofmulti-walled carbon nanotubes (Nanocyl-7000 manufactured by Nanocyl,average diameter: 10 nm (the average diameter was a value obtained byarithmetically averaging values measured at 200 or more sites throughthe use of an image photographed with a scanning electron microscope))with respect to pure water manually (first mixing step); and thenloading the mixture into a triple roll having a roll diameter of 50 mm(EXAKT M-50 I manufactured by Nagase Screen Printing Research Co., Ltd.)(roll temperature: 25° C. or more and 40° C. or less), followed bykneading for 3 minutes or more and 10 minutes or less (second mixingstep). A roll nip was 0.001 mm or more and less than 0.01 mm, roll speedratios V1, V2, and V3 were 1, 1.8, and 3.3, respectively, and the rollspeed V3 was 1.2 m/s in terms of a peripheral speed.

(1-3) Production of Reverse Osmosis Composite Membrane

The porous support having an area of 400 cm² was immersed in the thirdaqueous solution for 2 minutes, and was then slowly lifted so that asupport surface became vertical. Nitrogen was blown from an air nozzleagainst the support to remove an excess aqueous solution from itssurface, and then 50 ml of a n-hexane solution at 25° C. containing 0.1mass % of trimesoyl chloride was applied so that the support surface wascompletely wet. After the resultant had been left at rest for 1 minute,in order for an excess solution to be removed from the porous support,liquid draining was performed by vertically holding the support surfacefor 1 minute. After that, the porous support was washed with water at45° C. for 2 minutes. Thus, a reverse osmosis composite membrane ofExample 1 was obtained.

(1-4) Measurement of Carbon Nanotube Content

SII EXSTAR 6000 THERMAL ANALYZER TG/DTA6200 was used in the measurementof the content of the carbon nanotubes in a reverse osmosis membrane.The reverse osmosis composite membrane was sampled in an alumina pan,and was evaluated for its carbon nanotube content at a rate oftemperature increase of 10° C./min under an air atmosphere by utilizinga difference in thermal decomposition starting temperature between thepolyamide and the carbon nanotubes. The reverse osmosis membrane ofExample 1 contained 15.5 mass % of the carbon nanotubes.

(2) Production of Sample of Example 2

A sample of Example 2 was produced in the same manner as in Example 1except the step of producing the third aqueous solution of the (1-2). InExample 2, 491 g of a first aqueous solution obtained by adding 481 g ofdistilled water to 10 g of m-phenylenediamine, and stirring and mixingthe contents with a magnetic stirrer, and 9 g of a second aqueoussolution containing disentangled carbon nanotubes (the second aqueoussolution was identical to that of Example 1) were stirred and mixed witha magnetic stirrer to provide 500 g of a third aqueous solutioncontaining 2.0 mass % of m-phenylenediamine and 0.3 mass % of the carbonnanotubes.

The carbon nanotube content of the sample of Example 2 was measured inthe same manner as in the (1-4). The reverse osmosis membrane of Example2 contained 12.5 mass % of the carbon nanotubes.

(3) Production of Sample of Example 3

A sample of Example 3 was produced in the same manner as in Example 1except the step of producing the third aqueous solution of the (1-2). InExample 3, 486.5 g of a first aqueous solution obtained by adding 476.5g of distilled water to 10 g of m-phenylenediamine, and stirring andmixing the contents with a magnetic stirrer, and 13.5 g of a secondaqueous solution containing disentangled carbon nanotubes (the secondaqueous solution was identical to that of Example 1) were stirred andmixed with a magnetic stirrer to provide 500 g of a third aqueoussolution containing 2.0 mass % of m-phenylenediamine and 0.45 mass % ofthe carbon nanotubes.

The carbon nanotube content of the sample of Example 3 was measured inthe same manner as in the (1-4). The reverse osmosis membrane of Example3 contained 17.5 mass % of the carbon nanotubes.

(4) Production of Sample of Example 4

A sample of Example 4 was produced in the same manner as in Example 1except the step of producing the third aqueous solution of the (1-2). InExample 4, 485 g of a first aqueous solution obtained by adding 475 g ofdistilled water to 10 g of m-phenylenediamine, and stirring and mixingthe contents with a magnetic stirrer, and 15 g of a second aqueoussolution containing disentangled carbon nanotubes (the second aqueoussolution was identical to that of Example 1) were stirred and mixed witha magnetic stirrer to provide 500 g of a third aqueous solutioncontaining 2.0 mass % of m-phenylenediamine and 0.5 mass % of the carbonnanotubes.

The carbon nanotube content of the sample of Example 4 was measured inthe same manner as in the (1-4). The reverse osmosis membrane of Example4 contained 20.0 mass % of the carbon nanotubes.

(5) Production of Sample of Example 5

A sample of Example 5 was produced in the same manner as in Example 1except the step of producing the third aqueous solution of the (1-2). InExample 5, 496.7 g of a first aqueous solution obtained by adding 486.7g of distilled water to 10 g of m-phenylenediamine, and stirring andmixing the contents with a magnetic stirrer, and 3.3 g of a secondaqueous solution containing disentangled carbon nanotubes (the secondaqueous solution was identical to that of Example 1) were stirred andmixed with a magnetic stirrer to provide 500 g of a third aqueoussolution containing 2.0 mass % of m-phenylenediamine and 0.11 mass % ofthe carbon nanotubes.

The carbon nanotube content of the sample of Example 5 was measured inthe same manner as in the (1-4). The reverse osmosis membrane of Example5 contained 5.0 mass % of the carbon nanotubes.

(6) Production of Sample of Example 6

A sample of Example 6 was produced in the same manner as in Example 1except the step of producing the third aqueous solution of the (1-2). InExample 6, 494 g of a first aqueous solution obtained by adding 484 g ofdistilled water to 10 g of m-phenylenediamine, and stirring and mixingthe contents with a magnetic stirrer, and 6 g of a second aqueoussolution containing disentangled carbon nanotubes (the second aqueoussolution was identical to that of Example 1) were stirred and mixed witha magnetic stirrer to provide 500 g of a third aqueous solutioncontaining 2.0 mass % of m-phenylenediamine and 0.2 mass % of the carbonnanotubes.

The carbon nanotube content of the sample of Example 6 was measured inthe same manner as in the (1-4). The reverse osmosis membrane of Example6 contained 9.0 mass % of the carbon nanotubes.

(7) Measurement of Permeate Flux and NaCl Rejection Rate

An aqueous solution of NaCl having a pH of 7, a temperature of 23° C.,and a concentration of 2,000 ppm was supplied to each of the reverseosmosis composite membranes of Examples 1 to 6 with a cross-flow testcell apparatus (TOSC Co., Ltd., SEPA CFII) at an operating pressure of1.55 MPa, and a filtration treatment was performed over 4 hours. Theelectric conductivities of the supplied water and permeated waterobtained by the filtration treatment were measured with an electricconductivity meter (ES-71) manufactured by HORIBA, Ltd. to provide apractical salinity. A NaCl rejection rate (%) was determined from a NaClconcentration obtained by converting the practical salinity by using thefollowing equation.

NaCl rejection rate (%)=100×{1-(NaCl concentration in permeatedwater/NaCl concentration in supplied water)}

In addition, the amount of the permeated water obtained by 24 hours ofthe filtration treatment was converted into a water permeability (m³)per an area of the membrane surface of 1 m², per 1 day, and per anoperating pressure of 1 MPa, and was determined as a permeate flux(m³/(m²·day·MPa)).

The results are shown in Table 1 and the graph of FIG. 5. In FIG. 5, anaxis of abscissa indicates the permeate flux (m³/(m²·day·MPa)) and anaxis of ordinate indicates the NaCl rejection rate (%).

As comparative examples, a reverse osmosis composite membrane formedonly of polyamide (Comparative Example 1) manufactured at the samepolyamide concentration (no carbon nanotubes) as that of theabove-mentioned third aqueous solution, and the reverse osmosiscomposite membranes of Non Patent Literatures 1 and 2 were subjected tothe measurement under the same conditions. The measured values are shownin Table 1 and FIG. 5. In FIG. 5, a point represented by an unfilledcircle corresponds to the measured values of Example 1, a pointrepresented by an asterisk corresponds to the measured values ofComparative Example 1, a point represented by an unfilled diamondcorresponds to the measured values of Non Patent Literature 1 (Kim etal.), a point represented by an unfilled square corresponds to themeasured values of Non Patent Literature 2 (Lee et al.), and a pointrepresented by an unfilled triangle corresponds to the measured valuesof Non Patent Literature 3 (Zhao et al.).

TABLE 1 CNT content NaCl rejection rate Permeate flux (mass %) (%)(m³/m²d MPa) Example 1 15.5 99.8 1.4 Example 2 12.5 99.8 1.1 Example 317.5 99.8 1.25 Example 4 20.0 99.8 1.3 Example 5 5.0 99.8 0.9 Example 69.0 99.8 1.0 Comparative 0.0 99.8 0.7 Example 1 Non Patent 98.5 0.69Literature 1 Non Patent 95.7 0.82 Literature 2 Non Patent 90.0 0.42Literature 3

(8) Chlorine Resistance

Each of the reverse osmosis composite membranes of Examples 1 to 6 andthe commercial reverse osmosis composite membrane used in the (7) wasimmersed in an aqueous solution of sodium hypochlorite having a pH of9±0.5, a temperature of 23° C., and a concentration of 200 ppm for 24hours, and then its permeate flux and NaCl rejection rate were measuredin the same manner as in the (7). Changes in NaCl rejection rate andpermeate flux calculated by using the measurement results from theexpression “100×(measured value after immersion)/(measured value beforeimmersion)” are illustrated in FIG. 6.

In FIG. 6, an axis of abscissa indicates the product of a chlorineconcentration and an immersion time (active chlorine capacity (ppm-h)),a left axis of ordinate indicates the NaCl rejection rate (the measuredvalues are represented by solid lines) (normalized rejection), and aright axis of ordinate indicates the permeate flux (the measured valuesare represented by broken lines) (normalized flux). In FIG. 6, a pointrepresented by a filled diamond corresponds to each of Examples 1 to 4,a point represented by a filled square corresponds to Example 5, a pointrepresented by x corresponds to Example 6, a point represented by afilled circle corresponds to Comparative Example 1, and a pointrepresented by a filled triangle corresponds to a commercial product (acommercial RO membrane PMHA (product name) manufactured by TorayIndustries, Inc.). The respective measurement results are normalizedwhile the NaCl rejection rate and the permeate flux at the time of theinitiation of the measurement (0 ppm·h) are each defined as 100%.

Each of the reverse osmosis composite membranes of Examples 1 to 4showed no changes in permeate flux and NaCl rejection rate during a timeperiod from the initiation of the measurement (0 ppm·h) to thecompletion of the measurement 24 hours after the initiation (4,800ppm·h), and hence the increase rate of the permeate flux remained at 0%and the reduction rate of the NaCl rejection rate also remained at 0%.

In addition, as can be seen from the results of the (7), the reverseosmosis composite membrane of Example 1 had a permeate flux and a NaClrejection rate higher than those of the reverse osmosis compositemembranes of Non Patent Literatures 1 to 3, though the membrane wasinferior in permeate flux to the commercial reverse osmosis compositemembrane. It is probably because the carbon nanotubes were notdisentangled and hence the carbon nanotubes were in an agglomerated fineparticle state that the NaCl rejection rate of each of the reverseosmosis composite membranes of Non Patent Literatures 1 to 3 was low.

In addition, as can be seen from the results of the (8), each of thereverse osmosis composite membranes of Examples 1 to 4 was excellent inchlorine resistance, and was able to maintain its NaCl rejection rateand permeate flux before the washing with chlorine even after thewashing. Each of the reverse osmosis composite membranes of Examples 5and 6 was superior in chlorine resistance to the reverse osmosiscomposite membrane of Comparative Example 1 and the commercial product,and was able to suppress its NaCl rejection rate before the washing withchlorine from reducing after the washing and to suppress its permeateflux before the washing from increasing after the washing.

(9) Electron Microscope Observation

FIG. 7 is a photograph of a torn surface of the reverse osmosiscomposite membrane 100 of Example 1 obtained with a scanning electronmicroscope. The surface of the porous support 102 was covered with thereverse osmosis membrane 104. The thickness of the reverse osmosismembrane 104 was about 100 nm. As illustrated in FIG. 7, thethree-dimensional structure of the carbon nanotubes 110 was able to beobserved in the reverse osmosis membrane 104. Specifically, the carbonnanotubes 110 had a continuous network structure (through thecrosslinked aromatic polyamide layers 112 covering the carbon nanotubes110) in a torn surface. In addition, when the surface of the reverseosmosis membrane of Example 1 was observed with a scanning electronmicroscope, no agglomerate of the carbon nanotubes was found.

FIG. 8 is a photograph of the carbon nanotube 110 of the reverse osmosiscomposite membrane 100 of Example 1 obtained with a transmissionelectron microscope. The carbon nanotube 110 extended in the left andright directions of the figure, and the crosslinked aromatic polyamidelayer 112 was formed around (above and below) the carbon nanotube 110. Aphotograph obtained by subjecting the crosslinked aromatic polyamidelayer 112 to electron diffraction analysis is shown in a portionsurrounded by a white frame. The crosslinked aromatic polyamide layer112 was molecularly oriented. The crosslinked aromatic polyamide layer112 may be molecularly oriented as a result of a π-π interaction betweena π-electron of the carbon nanotube 110 and a n-electron of thecrosslinked aromatic polyamide. In addition, according to FIG. 8, thesurface of the carbon nanotube 110 forming the three-dimensionalstructure was covered with the crosslinked aromatic polyamide layer 112having a thickness of about 5 nm.

In addition, for comparison, a transmission electron microscopephotograph of the crosslinked aromatic polyamide in Comparative Example1 serving as the reverse osmosis membrane free of any carbon nanotube isillustrated in FIG. 9. A portion surrounded by a white frame is a resultof electron diffraction analysis and the crosslinked aromatic polyamidewas not molecularly oriented.

Closest distances between the carbon nanotubes were measured for thesample of Example 1. The results are illustrated in FIG. 3. A thinfilm-like test piece in which the surface of the reverse osmosismembrane was turned into a smooth surface was cut out through cuttingalong the surface of the reverse osmosis composite membrane of Example 1by a cryo-microtome method, and the smooth surface (reverse osmosismembrane) of the test piece was observed with a scanning electronmicroscope. A plurality of observation points (points along the vicinityof the central axis of a fiber) were prepared for each of the carbonnanotubes in a scanning electron microscope image, and a distancebetween observation points at positions closest to each other in theobservation points of adjacent carbon nanotubes was measured as aclosest distance. FIG. 3 is a graph in which the measurement results areplotted as filled triangle marks against an axis of abscissa indicatingthe closest distance (nm) and an axis of ordinate indicating the numberof measurement points (frequency). A measurement area and the number ofmeasurement points in the test piece were 441 μm² and 20,000,respectively.

As illustrated in FIG. 3, the graph showing the frequency of the closestdistance showed the following normal distribution: the distribution hada peak (around 60 nm) within the range of the thickness (100 nm) of thereverse osmosis membrane, and the half-width (60 nm) of the peak wasequal to or less than the thickness (100 nm) of the reverse osmosismembrane.

(10) Production and Observation of Sample of Comparative Example 2

In Comparative Example 2, 477.6 g of distilled water and 0.4 g of sodiumdodecyl sulfate serving as a surfactant were added to 2.0 g of carbonnanotubes, and the contents were mixed and stirred with a magneticstirrer for 30 minutes. The solution was subjected to an ultrasonictreatment in an ultrasonic treatment bath for 6 hours to provide asecond aqueous solution. 10 g of m-phenylenediamine was added to thesecond aqueous solution after the ultrasonic treatment, and the contentswere stirred and mixed with a magnetic stirrer to provide 500 g of athird aqueous solution containing 2.0 mass % of m-phenylenediamine, 0.4mass % of the carbon nanotubes, and 0.08 mass % of the surfactant. Areverse osmosis composite membrane sample of Comparative Example 2 wasproduced in the same manner as in Example 1 except the step of producingthe third aqueous solution.

A permeate flux and a NaCl rejection rate were measured by using thesample of Comparative Example 2 in the same manner as in the (7). Thesample of Comparative Example 2 had a permeate flux of 8.3m³/(m²·day·MPa) and a NaCl rejection rate of 25%.

FIG. 10 is a photograph obtained by photographing the entirety of thesample (200 mm×200 mm) of Comparative Example 2, and FIG. 11 is aphotograph obtained by enlarging part of FIG. 10. Many small blackpoints in FIG. 11 are agglomerates of the carbon nanotubes and were ableto be visually observed.

Closest distances between the carbon nanotubes were measured for thesample of Comparative Example 2 in the same manner as in Example 1, anda graph was created by plotting the results as filled circle marks inFIG. 3. As illustrated in FIG. 3, a peak showing a normal distributionwas not obtained within the range of the thickness of the reverseosmosis membrane despite the fact that Example 1 and Comparative Example2 contained the same amount of the carbon nanotubes. No closestdistances were measured for the carbon nanotubes in each of theagglomerates.

The reverse osmosis composite membrane of Comparative Example 2 wasincreased in permeate flux and reduced in NaCl rejection rate probablybecause the carbon nanotube agglomerate portions passed the aqueoussolution without rejecting NaCl. Substantially no crosslinked polyamidemembranes may be formed in the agglomerate portions. It was found thatunless the carbon nanotubes were disentangled, the reverse osmosiscomposite membrane could not function as a reverse osmosis membrane.

(11) Antifouling Property

A permeate flux when each of the reverse osmosis composite membranes ofExample 1 and Comparative Example 1, and the commercial reverse osmosiscomposite membrane was brought into contact with an aqueous solutioncontaining bovine serum albumin at a concentration of 200 ppm for 96hours was measured. A change rate with time when a permeate flux at thetime of the initiation of the measurement (0 h) is defined as 100% isillustrated in FIG. 12.

In FIG. 12, points represented by filled diamonds correspond to themeasured values of the change rate of the permeate flux of Example 1,points represented by filled triangles correspond to the measured valuesof the change rate of the permeate flux of Comparative Example 1, andpoints represented by filled squares correspond to the measured valuesof the change rate of the permeate flux of the commercial product.

The reverse osmosis composite membrane of Example 1 was excellent inantifouling property because its permeate flux showed a small changeabout 20 hours after the initiation of the measurement and remainedsubstantially unchanged about 40 hours after the initiation, and thechange rate of the permeate flux 72 hours after the initiation was lessthan 35%.

REFERENCE SIGNS LIST

100: Reverse osmosis composite membrane, 102: Porous support, 104:Reverse osmosis membrane, 110: Carbon nanotube, 112: Crosslinkedaromatic polyamide layer, 120: Crosslinked aromatic polyamide

1. A reverse osmosis composite membrane, comprising: a porous support;and a reverse osmosis membrane arranged on the porous support andcontaining a crosslinked polyamide and carbon nanotubes, the reverseosmosis membrane containing disentangled carbon nanotubes in thecrosslinked polyamide; a distribution of closest distances between thecarbon nanotubes in the reverse osmosis membrane having a peak that iswithin a range of a thickness of the reverse osmosis membrane; and ahalf-width of the peak being equal to or less than the thickness of thereverse osmosis membrane.
 2. The reverse osmosis composite membraneaccording to claim 1, wherein the disentangled carbon nanotubes form acontinuous three-dimensional structure through the crosslinkedpolyamide.
 3. The reverse osmosis composite membrane according to claim1, wherein the crosslinked polyamide is a crosslinked aromaticpolyamide; and wherein the reverse osmosis membrane contains amolecularly-oriented crosslinked aromatic polyamide configured to coverthe carbon nanotubes.
 4. The reverse osmosis composite membraneaccording to claim 1, wherein a content of the carbon nanotubes in thereverse osmosis membrane is 5 mass % or more and 30 mass % or less. 5.The reverse osmosis composite membrane according to claim 1, wherein thedistribution is a normal distribution.
 6. The reverse osmosis compositemembrane according to claim 1, wherein the carbon nanotubes have anaverage diameter of 5 nm or more and 30 nm or less.
 7. The reverseosmosis composite membrane according to claim 3, wherein themolecularly-oriented crosslinked aromatic polyamide forms a layer on asurface of each of the carbon nanotubes, the layer having a thickness of1 nm or more and 50 nm or less.
 8. The reverse osmosis compositemembrane according to claim 1, wherein the reverse osmosis compositemembrane has a permeate flux of 0.8 m³/(m²·day·MPa) or more and a NaClrejection rate of 95% or more when an aqueous solution of NaCl having apH of 7, a temperature of 23° C., and a concentration of 2,000 ppm issupplied at an operating pressure of 1.55 MPa; and wherein the reverseosmosis composite membrane has a reduction rate of the NaCl rejectionrate of less than 10% after having been immersed in an aqueous solutionof sodium hypochlorite having a pH of 9±0.5, a temperature of 23° C.,and a concentration of 200 ppm for 24 hours.
 9. The reverse osmosiscomposite membrane according to claim 1, wherein the reverse osmosiscomposite membrane has a reduction rate of a permeate flux of less than35% after having been brought into contact with an aqueous solutioncontaining bovine serum albumin at a concentration of 200 ppm for 72hours.
 10. A method of manufacturing a reverse osmosis compositemembrane, comprising: bringing a mixed liquid containing carbonnanotubes, water, and an amine component into contact with a poroussupport; and then subjecting the amine component in the mixed liquidadhering to the porous support to a crosslinking reaction, the mixedliquid being produced through a step of pressurizing and compressing anaqueous solution containing the carbon nanotubes while flowing theaqueous solution, followed by release or reduction of a pressure toreturn a volume of the aqueous solution to an original volume to mix thecarbon nanotubes.
 11. The method of manufacturing a reverse osmosiscomposite membrane according to claim 10, wherein a content of the aminecomponent in the mixed liquid is 1.0 mass % or more and 3.0 mass % orless; and wherein a content of the carbon nanotubes in the mixed liquidis 0.11 mass % or more and 1.3 mass % or less.
 12. The method ofmanufacturing a reverse osmosis composite membrane according to claim10, wherein the amine component is an aromatic amine.
 13. The method ofmanufacturing a reverse osmosis composite membrane according to claim10, wherein the aqueous solution containing the carbon nanotubes furthercontains a surfactant.